1. Field of the Invention
This invention relates to a marine riser safety valve which is also known as a marine riser fill-up valve. More specifically, a marine riser fill-up valve prevents collapse of a riser if the level of drilling fluid suddenly drops because of loss of circulation or displacement of the mud column in the riser by formation gas. Still more specifically, the valve, placed in the riser string, is intended to sense the difference between the sea water pressure at the depth in which the valve is placed and the drilling fluid pressure in the interior of the valve (riser). Upon actuation of the valve, sea water is allowed to enter the interior of the riser thereby preventing riser collapse due to the high pressure hydrostatic head of the sea water on the exterior of the riser and a loss of sufficient hydrostatic head of drilling fluid in the riser.
2. Description of the Prior Art
A prior art marine riser fill-up valve is described at page of 6857 of the composite catalog of oil field equipment and services, 1978-79 version, Volume 4. The riser fill-up valve described there is manufactured by the Vetco Company and includes a sensing valve which senses differential pressure between the outside of the body of the valve and its interior bore. The valve hydraulically actuates a sliding port cover to uncover fill-up ports in the event of a sudden differential pressure between the outside of the valve and the interior bore. The riser fill-up valve described above has a disadvantage in that it requires complex hydraulic control circuits to assure its effective automatic operation. Also the pressure sensor may be actuated by a relatively short time interval of a differential pressure condition as may happen during a swabbing operation of the riser. In addition, the small sensing chamber on the pressure sensor may readily become fouled with drilling mud or cement. Another disadvantage of the prior art riser fill-up valve described above is that it is costly to manufacture, has complex mechanisms to make it work and has too much inertia to overcome when cycling of the valve is required.
3. Identification of Objects of the Invention
It is an object of the invention to be described below to provide a marine riser fill-up valve which is of simple construction and affords automatic operation with only a minimum of opportunity for malfunction.
It is another object of the invention to provide a marine riser fill-up valve which is less massive than the prior art riser fill-up valves and is more economical to manufacture.
It is another object of the invention to provide manual override controls for the riser fill-up valve which will enable the valve to be opened or closed on command of an operator.
It is another object of the invention to provide an automatic riser fill-up valve which can be used as a riser dump valve by operating manual controls.
It is another object of the invention to provide a pressure differential sensor which automatically actively actuates the valve, and if actuation somehow is prevented, will rupture before the pipe collapses thereby preventing collapse of the riser itself.
It is another object of the invention to provide a marine riser fill-up valve which can be pressure tested for operability before installation in a marine riser string without the need for a pressure tank.
It is another object of the invention to provide a marine riser fill-up valve which is less sensitive to mud plugging of fill-up ports because of the action of the pressure differential sensor itself and the large volume of the mud chamber and its capacity to drain readily.